A basic commercial procedure for recovery of hydrogen sulfide from acid gas streams is the Claus process. The basic chemical reactions occurring in the Claus process are presented in Equations (1), (2), and (3). EQU H.sub.2 S+1/2O.sub.2 =H.sub.2 O+S (1) EQU H.sub.2 S+3/2O2=H.sub.2 O+SO.sub.2 ( 2) EQU 2H.sub.2 S+SO2=2H.sub.2 O+3S (3)
Reactions (1) and (2) occur in a thermal zone (reaction furnace) and reaction (3) occurs in a catalytic zone (catalytic reactor). The Claus process thus comprises two stages: (1) a thermal stage above, for example, about 1000.degree. F., and typically in the range of about 2000.degree. F. to about 3000.degree. F., and (2) a catalytic stage, generally, for example, between a temperature somewhat above the sulfur dew point of the gas stream and about 700.degree. F.
In the thermal zone, a gas stream containing hydrogen sulfide can be burned with atmospheric oxygen in a reaction furnace, typically as indicated above in the range of about 2000.degree. F. to about 3000.degree. F., to form hot combustion gases containing a substantial amount of free sulfur which can be condensed after cooling the hot combustion gases, for example, first in a waste heat boiler and subsequently in a sulfur condenser.
The lean gas stream leaving the sulfur condenser, having a reduced concentration of sulfur species, can be reheated before being passed to a first catalytic reactor operated above the dew point of sulfur, for example, by recombining a controlled portion of hot combustion gases from the reaction furnace, with or without partial cooling with such reaction gases. The recombining can, for example, be effected via a bypass reheat line and valve, which can fail due to the necessity of operating under very severe conditions.
Such reheating of the gas stream in the Claus process prior to catalytic conversion is necessary to maintain the temperature of the gas stream flowing through the first, and subsequent, catalytic reactor(s) above the dew point of sulfur because condensation of sulfur can lead to rapid catalyst deactivation. The effluent gas streams containing free sulfur leaving each catalytic reactor can be then again cooled and sulfur condensed. The process of reheating, catalytically reacting, and sulfur condensing, may be repeated in one, two, or even three or more additional catalytic stages operated above the dew point of sulfur. As conversion of hydrogen sulfide to sulfur occurs in the catalytic stages and more sulfur is removed from the gas stream, the dew point of sulfur of the gas stream is reduced, permitting operation at lower temperatures, thus improving conversion. After leaving the last sulfur condenser, the gas stream, which may still contain appreciable amounts of sulfur compounds and some sulfur vapor, is either incinerated to convert all sulfur compounds to sulfur dioxide before venting to the atmosphere, or further treated in a separate process for removal of residual sulfur, such as, for example, by the so-called Cold Bed Adsorption (CBA) process, or other processes for tail-gas clean-up.
In the CBA (Cold Bed Adsorption) process, for example, typically as described in the U.S. Pat. Nos. 3,702,844, 3,758,676, 3,749,762, and 4,035,474, the hydrogen sulfide and sulfur dioxide content of Claus plant effluent streams is decreased by conversion to elemental sulfur in the presence of a Claus-type catalyst at a temperature effective for adsorbing a preponderance of the thus produced sulfur on the catalyst, typically, for example, between about 270.degree. and 300.degree. F. Since the Claus reaction is a reversible exothermic reaction and since the chemical process that occurs in the reactor can be viewed as an approach to chemical equilibrium, the lower temperatures associated with the CBA reactor have in principle two advantages over the higher temperature Claus reactor, each of which contributes to lower reactant concentrations and more efficient removal of sulfur. First, the temperature dependence of the thermodynamic equilibrium constant of an exothermic reaction favors lower reactant concentrations as the temperature decreases and the CBA reactors are operated at relatively low temperatures compared to the basic Claus process described above. Second, the particular temperature range of the CBA reactor is below the dew point of sulfur, so that physical deposition of a preponderance of the reaction product (free sulfur) as an adsorbed phase occurs. The CBA reactor unlike the high-temperature Claus reactor must, however, be periodically regenerated by vaporizing the deposited sulfur with a hot stripping or regeneration gas stream followed by cooling the reactor back to the desired operating temperature for adsorption, so that a complete sulfur recovery cycle for a given CBA reactor in more or less continuous operation includes an adsorption phase, a regeneration phase, and a cooling phase.
Various specific methods of regenerating the sulfur-laden catalyst of low temperature reactors involving a variety of proposed flow schemes have been suggested and implemented with varying degrees of commercial success. All have in common a basic heat energy requirement for vaporizing or desorbing sulfur from the catalyst.